Data collected at MacCHESS under express mode proposal EM188 were used in the structure determination of the DPC4 and retinoblastoma (Rb) tumor suppressor proteins. DPC4 (now also known as Smad4) signals from TGFb superfamily receptor Ser/Thr protein kinases at the cell surface to the nucleus through homo- and hetero-oligomeric interactions. The crystal structure of the C-terminal domain (CTD) of DPC4, determined at 2.0 resolution, revelas that the DPC4-CTD forms a crystallographic trimer through a conserved protein-protein interface to which the majority of the tumor-derived missense mutations map. We showed that these mutations disrupt homo-oligomerization in vitro and in vivo, and suggested the hypothesis that the trimeric assembly of the Smad4/DPC4-CTD is a critical function in signaling that is targeted by tumorigenic mutations. Because our paper describing the DPC4 structure (1) went in press while we were collecting data at MacCHESS, it only describes a 2.5-refined structure. The coordinates submitted to the Brookhaven Protein Data Bank were refined using the 2.0 data collected at MacCHESS. The retinoblastoma tumor suppressor, which in most human cancers is inactivated by mutations, is instead inactivated by the human papilloma virus E7 oncoprotein in cervical cancer. The crystal structure of Rb bound to a nine residue E7 peptide containing the LxCxE motif, shared by other Rb-bidning viral and cellular proteins, was determined at 1.85 resolution (2). The structure shows that the LxCxE peptide binds a highly conserved groove on the B box or the interface with the A box, which the structure shows is required for the stable folding of the B box.